(1) Field of the Invention
This invention relates to an improvement in the purification of an aqueous acrylamide solution which has been obtained by the catalytic hydration of acrylonitrile with water in the presence of a copper-containing catalyst. The invention particularly concerns with a method for preparing an aqueous acrylamide solution useful as the starting material of a coagulating agent.
(2) Description of the Prior Art
Acrylamide is useful as a starting material of polyacrylamide which finds a wide variety of uses such as paper strengthening agent, coagulating agent, soil reforming agent and the like.
Acrylamide has been recently produced by catalytic hydration of acrylonitrile with water in the presence of a copper-containing catalyst. That process is extremely advantageous, because the product is obtained in the form of an aqueous solution and can be subjected to the polymerization reaction as is. If the aqueous acrylamide solution obtained upon the hydration is directly subjected to the polymerization reaction, however, high quality polyacrylamide cannot be obtained, probably due to the trace substances present in the solution, such as
(1) unreacted acrylonitrile, PA1 (2) metallic ions such as copper ion and complex ion formed with the copper, which are eluted from the catalyst component, PA1 (3) impurities contained in the starting acrylonitrile and PA1 (4) side reaction products such as an organic acid.
Of those substances, unreacted acrylonitrile can be easily removed by the practices known per se, such as distillation. Also the copper forming the metallic ion and copper complex ion can be removed, for example, by a treatment with H-form, or an ammonium salt form, strongly acidic cation exchange resin. It is difficult, however, to completely eliminate the organic matters forming the complex ions with copper, the traces of side reaction products and the impurities contained in the starting acrylonitrile, with such a strongly acidic cation exchange resin treatment alone.
As the means to cover the above shortcomings, three methods have been proposed. The first method proposes to treat the aqueous acrylamide solution obtained upon the catalytic hydration of acrylonitrile with a OH-form or a weak acid salt form, strongly basic anion exchange resin (Japanese Laid-Open Patent Publication No. 82011/1975). As the second method, it is known to treat the so obtained aqueous acrylamide solution with a mixed bed composed of a cation exchange resin and a strongly basic anion exchange resin. As the strongly basic anion exchange resin, "Diaion
316" (commercial product of Mitsubishi Kasei Kogyo K.K.), which is strongly basic, porous I form, is recommended (Japanese Laid-Open Patent Publication No. 83323/1975). As the third method, furthermore, it is known to treat the aqueous solution first with a strongly acidic cation exchange resin and then with a weakly basic anion exchange resin, and as the useful exchange resin for the second stage treatment, those containing primary, secondary, or tertiary amino groups are named (Japanese Laid-Open Patent Publication No. 91819/77). The literature disclosing the third method gives the experimental results using, as such amino group-containing resins, "Lewatit MP 62" (commercial product of Bayer A.G.) and "Diaion WA 10" (commercial product of Mitsubishi Kasei Kogyo K.K.), and names "Amberlite IRA-93" (commercial product of Rohm and Haas Co.) as an example. The exchange groups of those resins, however, are invariably tertiary amino groups. Thus absolutely no disclosure is found in the literature concerning the use of weakly basic anion exchange resins containing primary and/or secondary amino groups.
According to our studies, the aqueous acrylamide solution which has been purified by such known methods as above is quite satisfactory for making the polyacrylamide for a paper strengthening agent. The solution cannot be satisfactorily purified, however, to serve as the starting material of polyacrylamide as a coagulating agent, for example, and can hardly provide a coagulating agent showing good coagulating ability and water-solubility.